In order to improved the mechanical properties, the heat resistance, and the like of a thermoplastic resin, a layered clay mineral such as talk and mica have been conventionally used as a filler (see Japanese Publication for Opposition No. 49-18615, Japanese Laid-Open Publication No. 55-16049, and Japanese Publication for Opposition No. 63-53222, for example). By adding a large amount of such layered clay mineral to a thermoplastic resin, the elastic modulus and the heat resistance of the resultant resin composition improves. However, a molded product obtained from the resin composition has the disadvantages of poor appearance, increased specific gravity, and poor color tone. Moreover, the strength, the tenacity, and the like of the resin composition are reduced due to insufficient dispersion of the layered clay mineral or insufficient adhesion between the layered clay mineral and the resin.
A surface treatment agent has generally been used for improving the adhesion between a filler and a resin to prevent the reduction of strength, impact strength, and the like of a resin composition. For example, a composite of a layered clay mineral treated with a silane coupling agent and a polyester resin is disclosed (see Japanese Laid-Open Publication No. 51-24653 and Japanese Laid-Open Publication No. 51-24654, for example). However, the conventional surface treatment method has not yet provided a sufficient effect of improving the strength and the like of the resin composition. Further, even if the surface of the layered clay was treated, the layered clay mineral itself still has an aggregate structure. Therefore, the problems of poor appearance, increase in the specific gravity, and poor color tone of the resultant molded product remained unsolved.
The layered clay mineral normally has an aggregate structure where about a hundred to several thousands of unit layers each having a thickness of about 1 nm are stratified. Therefore, in the above prior art technique, the layered clay mineral dispersed in the thermoplastic resin exists, not as foliated layers, but in the state of the aggregate structure.
If such an aggregate structure of the layered clay mineral could be exfoliated to allow unit layers to be dispersed separately in the thermoplastic resin, the resultant resin would be strengthened by the addition of only a small amount of the layered clay mineral , and the problems of the poor appearance, the increase in the specific gravity, and the poor color tone of the resultant molded product would be overcome. However, no technique that can exfoliated the layered clay mineral to the state of foliated unit layers to disperse the unit layers in the thermoplastic resin uniformly by use of a silane-coupling agent so as to obtain a resin composition having excellent properties has yet been provided.